1.Oxbow, IMO, really is the best food you can give your boy.
2. Leafy greens can be started at 3 months old. Romaine and green/red lettuce are good options to start with. My boys go crazy for parsley and dill, so those are also good to try. A rule of thumb is 1 cup of greens per 2 pounds of bunny. My boys are 5 pounds, so I roughly give them 2-3 cups every night as their dinner. Remember to introduce greens slowly as some greens might not agree with his tummy. So do one green for a week and start with 1 leaf and increase to the recommended amount throughout the week, and the next week add in a second green, repeating this same process. My boys usually get a half box each of the Olivia’s Organic boxed greens/any boxed green. I would go easy on kale, cabbage, and spinach, as these can induce gas in some bunnies.
Fruits and sugary veggies like carrots or peppers should be given in limited quantities and introduced slowly at 6 months. Usually 2 tablespoons every other day (2-3 times a week basically) is a good treat guideline. I wouldn’t recommend dehydration because it’s harder to really gauge sugar intake. For example, 2tbsp of raisins is obviously way more sugar than 2 tbsp grapes. You can dehydrate them though and give like 1 dehydrated apple slice, or 1 dehydrated banana chip.
3. A healthy bunny starts with a health diet. As long as his pellets are under control, he’s consuming lots of hay, and eating veggies, he really shouldn’t need a supplement. A probiotic is good for a bunny who’s been on antibiotics or whatnot, but in general just a healthy diet is all he’ll need.